U.S. patent number 4,592,949 [Application Number 06/732,432] was granted by the patent office on 1986-06-03 for vinylidene fluoride resin split yarn, process for production thereof and filter comprising the same.
This patent grant is currently assigned to Kureha Kagaku Kogyo Kabushiki Kaisha. Invention is credited to Toshiya Mizuno, Naohiro Murayama, Yoshikichi Teramoto.
United States Patent |
4,592,949 |
Mizuno , et al. |
June 3, 1986 |
Vinylidene fluoride resin split yarn, process for production
thereof and filter comprising the same
Abstract
A vinylidene fluoride resin having an inherent viscosity of 0.8
to 1.4 dl/g in dimethylformamide is melt-extruded and cooled and
solidified under draft-stretching to form a film having a
birefringence of 25.times.10.sup.-3 or larger. The film is
heat-treated at 80.degree.-120.degree. C. and split into a split
yarn. The split yarn has a Young's modulus even higher than that of
the polypropylene split yarn and, when provided with a surface
charge above the glass transition temperature, retains the charge
semi-permanently. The charged split yarn is particularly suitable
for forming a gas filter.
Inventors: |
Mizuno; Toshiya (Iwaki,
JP), Teramoto; Yoshikichi (Iwaki, JP),
Murayama; Naohiro (Iwaki, JP) |
Assignee: |
Kureha Kagaku Kogyo Kabushiki
Kaisha (Tokyo, JP)
|
Family
ID: |
26430914 |
Appl.
No.: |
06/732,432 |
Filed: |
May 6, 1985 |
Foreign Application Priority Data
|
|
|
|
|
May 7, 1984 [JP] |
|
|
59-89498 |
May 7, 1984 [JP] |
|
|
59-89499 |
|
Current U.S.
Class: |
442/189; 428/364;
428/392; 428/394; 428/910; 442/301; 442/334; 442/414;
55/DIG.45 |
Current CPC
Class: |
A24D
3/08 (20130101); B01D 39/083 (20130101); B01D
39/1623 (20130101); C08J 5/18 (20130101); D01D
5/42 (20130101); D01F 6/12 (20130101); B01D
39/04 (20130101); Y10T 428/2913 (20150115); B01D
2239/04 (20130101); B01D 2239/1291 (20130101); C08J
2327/16 (20130101); Y10S 428/91 (20130101); Y10S
55/45 (20130101); Y10T 442/608 (20150401); Y10T
442/3976 (20150401); Y10T 442/3065 (20150401); Y10T
442/696 (20150401); Y10T 428/2964 (20150115); Y10T
428/2967 (20150115) |
Current International
Class: |
A24D
3/00 (20060101); A24D 3/08 (20060101); B01D
39/02 (20060101); B01D 39/08 (20060101); B01D
39/04 (20060101); C08J 5/18 (20060101); D01F
6/02 (20060101); D01D 5/42 (20060101); D01F
6/12 (20060101); D01D 5/00 (20060101); D03D
003/00 () |
Field of
Search: |
;428/224,229,364,392,394,910 |
References Cited
[Referenced By]
U.S. Patent Documents
|
|
|
4298719 |
November 1981 |
Mizuno et al. |
|
Primary Examiner: Bell; James J.
Attorney, Agent or Firm: Wenderoth, Lind & Ponack
Claims
What is claimed is:
1. A split yarn of a vinylidene fluoride resin characterized
by:
(a) having a crystal melting point of 182.degree. C. or higher as
measured at a temperature elevation speed of 10.degree. C./minute
in nitrogen atmosphere by a differential scanning calorimeter,
and
(b) comprising crystals giving an average crystal width of 200
.ANG. or larger.
2. The split yarn according to claim 1, which has a surface
charge.
3. The split yarn according to claim 1, which has a proportion of
heat absorption area based on the crystal melting point of
182.degree. C. or higher to the total heat absorption area of 20%
or more on a chart of the differential scanning calorimeter
measurement.
4. The split yarn according to claim 1, which comprises vinylidene
fluoride homopolymer, or a copolymer of vinylidene fluoride and a
monomer copolymerizable therewith comprising at least 70 mol. % of
vinylidene fluoride units.
5. The split yarn according to claim 4, wherein the vinylidene
fluoride homopolymer or copolymer has an inherent viscosity of 0.8
to 1.4 dl/g as measured in dimethylformamide at a concentration of
0.4 g/dl at 30.degree. C.
6. The split yarn according to claim 4, wherein the vinylidene
fluoride homopolymer or copolymer predominantly comprises
.beta.-phase crystals.
7. The split yarn according to claim 1, which comprises strips of
the vinylidene fluoride resin having a section with an average
shorter side length of 1-10 microns and an average longer side
length of 2-100 microns.
8. A filter comprising a gathering of the split yarn according to
claim 2.
9. The filter according to claim 8, which comprises a woven or
nonwoven cloth of the split yarn.
10. The filter according to claim 8, which comprises a mass of the
split yarn.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a split yarn of a vinylidene
fluoride resin, a process for production thereof and a filter
comprising the split yarn.
From a vinylidene fluoride resin, a filament yarn which is made of
continuous fiber has been commercially produced, whereas a split
yarn or slit yarn which is made of staple form fiber has not been
commercially produced.
Not only from a vinylidene fluoride resin, but also from many other
resins, it is difficult to obtain a split yarn. A polypropylene
yarn which is obtained by cold-stretching a polypropylene film at a
high stretching ratio of around 10 times and splitting the
stretched film has been known as a split yarn. However, no split
yarn has been obtained in a similar manner from any other resin.
This may be attributable to the fact that polypropylene has a high
crystallinity and also has a weak cohesion strength so that the
above described process is readily applicable thereto. For this
reason, it has been considered difficult to obtain a split yarn
from a vinylidene fluoride resin which has a low crystallinity and
a larger intermolecular cohesion strength by the splitting process
as described above.
A vinylidene fluoride resin however is a polar polymer and has a
weather resistance and a chemical resistance which are essentially
better than those of polypropylene. Accordingly, if a split yarn
thereof can be obtained, it is expected be a useful functional
material. Especially, if the split yarn of a vinylidene fluoride
resin is obtained in an electrically charged state, it would be an
excellent filter material retaining its charge stably.
SUMMARY OF THE INVENTION
A principal object of the present invention is, in view of the
above circumstances, to provide a split or slit yarn of a
vinylidene fluoride resin and a process for producing the same.
Another object of the present invention is to provide a charged
split yarn and a filter comprising the same.
According to our study, it has been discovered that a split yarn of
a vinylidene fluoride resin can be obtained if a vinylidene
fluoride resin having an appropriate molecular weight is processed
under appropriate conditions, and the thus obtained vinylidene
fluoride resin split yarn has characteristic crystal melting point
and crystal size.
The split yarn of a vinylidene fluoride resin according to the
present invention is based on the above discovery and more
specifically characterized by:
(a) having a crystal melting point of 182.degree. C. or higher as
measured at a temperature elevation speed of 10.degree. C./minute
in nitrogen atmosphere by a differential scanning calorimeter,
and
(b) comprising crystals giving an average crystal width of 200
.ANG. or larger.
The process for producing a split yarn of a vinylidene fluoride
resin, comprises the steps (a)-(d) in the order named:
(a) melt-extruding a vinylidene fluoride resin into a sheet or
tube, said vinylidene fluoride resin having an inherent viscosity
of 0.8 to 1.4 dl/g as measured in a dimethylformamide solution at a
concentration of 0.4 g/dl at 30.degree. C.,
(b) cooling and solidifying the extruded sheet or tube under
draft-stretching to form a film having a birefringence of
25.times.10.sup.-3 or larger,
(c) heat treating the film at a temperature of from 80.degree. to
180.degree. C., and
(d) splitting the film into a split yarn.
Furthermore, a charged split yarn is obtained by applying an
electric voltage to the film at any point from after the step (b)
to after the step (c) at a temperature which is not lower than the
glass transition temperature but is lower than the melting point of
the principal constituent resin of the vinylidene fluoride resin.
The filter according to the invention is constituted by a gathering
of the charged split yarns.
These and other objects, features and advantages of the present
invention will become more apparent upon a consideration of the
following description concluding with specific examples and
comparative examples taken in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows an example of melting curve obtained by a differential
scanning calorimeter measurement; and
FIG. 2 is a graph showing a relationship between Young's modulus
and birefringence for explaining draft stretching.
DETAILED DESCRIPTION OF THE INVENTION
The term "vinylidene fluoride resin" used herein for describing the
present invention is intended to cover a vinylidene fluoride
homopolymer; a vinylidene fluoride copolymer comprising as the
constituent units thereof 70 mol. % or more of vinylidene fluoride
and one or more monomer copolymerizable therewith such as ethylene,
vinyl fluoride, trifluoroethylene, tetrafluoroethylene,
trifluorochloroethylene, and hexafluoropropylene; and a vinylidene
fluoride resin composition comprising 80 wt. % or more thereof, of
such a vinylidene fluoride homopolymer or copolymer. Examples of
the components other than the vinylidene fluoride homopolymer or
copolymer include other polymers and additives such as
plasticizers, processing aids and ultraviolet absorbers. Examples
of the above-mentioned other polymers include polyethylene,
polypropylene, polyvinyl acetate, polymethyl methacrylate,
polyethyl methacrylate, polystyrene, aliphatic acid polyesters,
aromatic polyesters, polyamides, and fluoride-containing polymers
other than vinylidene fluoride homopolymer or copolymer.
In the present invention, the vinylidene fluoride resin, among the
above enumerated examples thereof, should preferably be a
vinylidene fluoride homopolymer or a copolymer of vinylidene
fluoride with a perfluoroolefin, particularly a copolymer of
vinylidene fluoride with tetrafluoroethylene, comprising 70 mol. %
or more, particularly 80 mol. % or more, of vinylidene fluoride
units.
The split yarn of the vinylidene fluoride resin according to the
present invention is characterized by having a crystal melting
point of approx. 182.degree. C. or higher as measured on heating at
a temperature elevation speed of 10.degree. C./min. in nitrogen
atmosphere by means of a differential scanning calorimeter
(hereinafter abbreviated as "DSC"). The crystal melting point is
mainly determined at a stage after the draft-stretching step in the
process for production of the split yarn. Herein the crystal
melting point refers to a temperature giving a heat absorption peak
in a melting curve obtained by the DSC measurement.
A split yarn can be produced more easily from such a resin that has
a larger proportion of the heat absorption area based on the
melting heat absorption peak of 182.degree. C. or higher occupied
in the total area of the heat absorption peaks on the DSC chart.
For example, a film of such a resin is more easily split to form a
split yarn having a larger Young's modulus. For this reason, the
proportion of the heat absorption area based on the melting heat
absorption peak of 182.degree. C. or higher in the total area of
heat absorption in the DSC chart should preferably be 20% or more,
more preferably 50% or more, further more preferably 80% or
more.
For the same reason, it is preferred that the resin constituting
the film has a crystal melting point at a higher temperature,
preferably 185.degree. C. or higher, more preferably 190.degree. C.
or higher. Besides, in the heat absorption area on the DSC chart,
the proportion occupied by the heat absorption area based on the
melting heat absorption peak of these temperatures or higher should
preferably be as high as possible, especially as explained above
with reference to the peak of 182.degree. C. or higher.
The base line for calculating the heat absorption area in the DSC
measurement chart is determinned as follows.
First, a sample to be measured is elevated in temperature to
220.degree. C. under the conditions as already described to
determine the melting curve of the sample. Then, after maintained
at 220.degree. C. for 30 seconds, the sample is lowered in
temperature at a rate of 10.degree. C./min. to be crystallized and
cooled to room temperature. The thus obtained melted and
recrystallized product is again elevated in temperature under the
same conditions to determine the crystal melting curve. The first
end point of crystal melting of the melting curve obtained, namely
the point when the heat generation peak has completed (T.sub.mid),
is determined. (See FIG. 1 obtained in an Example described
hereinafter.)
The melting peak in the melting curve of the measured sample is
divided at this point T.sub.mid, while both the ends on the maximum
temperature side and the minimum temperature free of any heat
absorption or evolution in the melting curve are respectively
connected to the point T.sub.mid to define a baseline. The
positions of both ends of no heat absorption or evolution are
determined by overlapping the DSC measurement chart obtained under
the state where no sample is contained, following otherwise
entirely the same conditions.
Another specific feature of the split yarn of the present invention
is that it comprises crystals giving an average crystal width of
the order of 200 .ANG. or larger, ordinarily within the range of
250 to 300 .ANG.. Herein, the average crystal width can be
determined by X-ray diffraction. Thus, a mass of a split yarn is
arranged generally in the direction of draft (stretching direction)
and hardened with an amorphous adhesive to form a sample in the
form of a film. The sample is placed so that its draft direction is
arranged longitudinally. An X-ray is incident on the film and the
intensity of reflection at the film surface, i.e., diffraction
intensity, of the X-ray in the horizontal direction is recorded on
a chart. When the average crystal width is obtained from the chart
thus obtained, a diffraction peak of the (110) plane is noted.
Thus, the diffraction intensity of the (110) plane is read on the
chart to determine the half-value width of the peak. On the other
hand, by use of silicon single crystal powder, the mechanical
expansion (namely, expansion of the diffraction peak inherent in
the measuring machine) is determined. The value obtained by
subtracting the half-value width of the mechanical expansion from
the half-value width of the measured sample is determined as the
true half-value width (.beta.w (radian)) of the sample. By use of
the true half-value width, the crystal length (L) is determined
from the Scherrer's equation:
where .theta. is the Bragg reflection angle of the measured
diffraction plane (i.e., the (110) plane), k is a constant (=1.0),
and .lambda. is the wavelength of X-ray CuK.alpha. (1.542A). (As to
details of such a measuring method, see, for example, "Basis of
X-ray crystallography", translated by Hirabayashi and Iwasaki,
Maruzen (published on Aug. 30, 1973), p. 569.) The measured values
described herein are those obtained by means of an X-ray
diffraction device produced by Rigaku Denki K.K. at a voltage of 40
KV and a current of 20 mA, with a slit system under the conditions
of a divergence split of 1.degree., a receiving slit of 0.3 mm in
diameter and a scattering slit of 1.degree. and at a scanning speed
of 2.theta.=1.degree./min. The X-ray is also monochromatized with
an Ni filter.
The above described split yarn of a vinylidene fluoride resin
according to the present invention may, for example, be produced in
the following manner.
First, a vinylidene fluoride resin having an inherent viscosity of
0.85 to 1.4 dl/g as measured as a dimethylformamide solution at a
concentration of 0.4 g/dl at 30.degree. C. is melt-extruded into a
sheet or tube. If the inherent viscosity is lower than the above
mentioned range, irregularity in thickness of the extrudate occurs
during the draft-stretching as will be described hereinafter so
that a high draft ratio cannot be attained to result in decrease in
mechanical strength. On the other hand, if the inherent viscosity
exceeds the above mentioned range, the extrusion and processing of
the vinylidene fluoride resin becomes difficult and, if the resin
is processed at a high temperature, it is liable to decompose
thermally. Preferably, a resin having an inherent viscosity
according to the above definition in the range of 0.9 to 1.3 dl/g,
particularly 1.0 to 1.2 dl/g is used.
The melt-extrusion may be carried out according to a known process
such as the T-die process or the inflation process. The resin may
preferably be first extruded through a circular inflation die and
then formed into a film by the inflation process. This is because
the T-die process is not desirable as it causes a large degree of
shrinkage in width so that both sides of the film cannot be
sufficiently oriented during the subsequent stretching at a high
draft ratio.
After the melt-extrusion, the extrudate is, as it is,
draft-stretched at a high draft ratio and simultaneously cooled to
solidify into a film. The draft ratio at this stage is so selected
as to provide a film having a birefringence of 25.times.10.sup.-3
or larger after cooling and solidification.
This is explained in rather detail hereinbelow. A highly
molecular-oriented film can be obtained by first melt-extruding the
vinylidene fluoride resin and then taking up the extruded film at a
certain high draft ratio while using a pinch or guide roller having
a temperature below the fastest crystallization temperature (i.e.,
the temperature giving the maximum crystallization speed) of the
principal resin (i.e., a resin having the largest content)
constituting the vinylidene fluoride resin and placed at an
environmental temperature below the fastest crystallization
temperature. At this time, the draft ratio can vary depending on
such factors as a melt-viscosity of the resin, environmental
temperature and a volume rate of cooling air, but it is set at such
a high level as to give a film having a birefringence .DELTA.n of
approx. 25.times.10.sup.-3 or larger. The thus taken-up film having
a large birefringence comprises crystals having a high melting
point as high as 182.degree. C. or higher, as different from a film
having a smaller birefringence, and also has a remarkably increased
Young's modulus as shown in FIG. 2, which shows a relationship
between .DELTA.n and Young's moduli for films which were obtained
by melt-extruding a vinylidene fluoride homopolymer having an
inherent viscosity of 1.1 dl/g through a circular die of 50 mm in
diameter and 2 mm in lip-clearance, inflating the extrudate at a
blow-up ratio of 1.2 and taking it up at various take-up speeds.
Herein, the operation of taking up a film at a draft ratio
exceeding such a critical draft ratio above which the Young's
modulus of a product film abruptly increases is referred to as
"draft-stretching".
The appropriate draft ratio depends on the melt viscosity of a
resin but is ordinarily of the order of 50 to 5000. When the melt
viscosity of a resin is low, a birefringence of 25.times.10.sup.-3
or larger for a cooled and solidified film cannot be obtained
unless the draft ratio is relatively increased. The draft ratio is
preferably so set as to give a birefringence of 30.times.10.sup.-3
or lager, particularly 35.times.10.sup.-3 or larger. This is
because the splitting as described hereinafter is easily carried
out.
When the inflation process is adopted, a blow-up ratio which is a
stretching ratio in the transverse direction should appropriately
be selected in addition to the draft ratio. The blow-up ratio is
ordinarily in the range of 0.7 to 2.5, and preferably in the range
of 0.8 to 1.5.
The cooling for solidification is conducted in a known manner. For
example, in the inflation process, a film may be extruded upwardly
and cooled by air, or may be extruded downwardly and cooled with a
cooling medium such as water as a typical example.
The thus obtained film is heat-treated at a temperature of
preferably 80.degree. to 180.degree. C. The heat treatment is
carried out in order to promote crystallization and is effectively
carried out around the fastest crystallization temperature of the
principal resin constituting the vinylidene fluoride resin. Thus,
the heat treatment is carried out at a temperature which is not
lower by 60.degree. C. and not higher by 60.degree. C. than the
fastest crystallization temperature, more preferably at a
temperature which is not lower by 40.degree. C. and not higher by
40.degree. C. than the fastest crystallization temperature.
The time for the heat treatment is preferably as long as possible
and is ordinarily at least 2 hours, more preferably 24 hours or
longer, at around the fastest crystallization temperature.
The film is not necessarily placed under a constant length state
but is preferably placed under a constant length state during the
heat treatment. By the heat treatment, the crystalline structure of
the vinylidene fluoride resin constituting the film is transformed
from one predominantly of the .alpha.-phase to one predominantly of
the .gamma.-phase.
After or simultaneously with the heat treatment, the film is
preferably cold-stretched to transform the crystalline structure to
one predominantly of the .beta.-phase, because the film is caused
to have a higher elasticity and can be formed into a split yarn
more easily thereby. The stretching ratio at this time may be of
the order of 1.1 to 5 times.
The thus obtained film is split into a split yarn comprising thin
fibrillated strips each having a rectangular section with an
average shorter side length of 1 to 10 microns and an average
longer side length of 2 to 100 microns. The splitting of the film
can be effected by using a roller around which needles are planted
as disclosed in Japanese Patent Laid-Open Application No.
180621/1983 and by causing the needle-planted roller to rotate
along the movement of the film and contact the film while
rotating.
The thus obtained split yarn may have a Young's modulus larger than
that of a known polypropylene split yarn. This is amazing in view
of the fact that the known filament of polyvinylidene fluoride has
a Young's modulus smaller than that of the polypropylene filament.
Further, the split yarn thus obtained according to the present
invention retains excellent features such as excellent
weatherability and chemical resistance which are inherent to known
vinylidene fluoride resins and are not attained by the
polypropylene split yarn.
The split yarn according to the present invention may be provided
with a surface charge. Herein, the surface charge of a split yarn
does not mean that the split yarn has a temporary surface charge
but means that it has a semi-permanent surface charge. More
specifically, the presence or absence of a surface charge is
determined in the following manner. Thus, a sample split yarn is
formed into woven or nonwoven cloth in a size of 10 cm.times.10 cm,
which is then placed in an environment of 23.degree. C. and 50%
humidity for 24 hours. The surface potential of the sample cloth is
measured under the same environmental conditions by means of a
rotating sector type surface potentiometer. The presence or absence
of a surface charge is judged by whether the arithmetic mean of 5
measured surface potentials for 5 sheets of such sample cloth is
above 1 V or not.
Such a split yarn of a vinylidene fluoride resin having a surface
charge is more excellent in dust-removal effect based on a surface
charge when compared with a polypropylene split yarn having a
surface charge and is particularly suitably used for a filter for
removing dust, smoke of cigarette, etc., in air. A filter otained
from the split yarn is not limited to such an air filter. Thus, the
filter is not only applicable for treatment of air but also
suitably applicable for any fluid assuming the form of gas or mist
at the time of treatment, particularly any kinds of gas.
Furthermore, the filter may also be applicable for a nonpolar
liquid.
The split yarn of a vinylidene fluoride resin of the present
invention having a surface charge as mentioned above may be
produced by applying a voltage to a film of a vinylidene fluoride
resin at an appropriate stage during the above described process
for producing a split yarn. The application of a voltage is carried
out at any time from after the step of draft-stretching and cooling
for solidification to before the splitting step, i.e., before,
simultaneously with, during or after the heat-treatment step,
preferably at a temperature not lower than the glass transition
temperature of and below the melting point of the principal resin
constituting the vinylidene fluoride resin. The voltage is applied
in the direction of thickness of the film at an intensity of 50
KV/cm or higher and below the dielectric breakdown intensity. The
application of a voltage is carried out for imparting a surface
charge to the split yarn. Therefore, while the voltage may be
applied during the progress of crystallization or after almost
complete crytallization in the film, it may preferably be carried
out after the heat-treatment step or started from an intermediate
point during the heat-treatment step. However, it is not
appropriate to apply the voltage to a split yarn for the purpose of
effective provision of the surface charge because a high electric
voltage cannot be applied thereto due to a low withstand voltage of
air in a gap between strips of the yarn.
The temperature for the voltage application should preferably be
within the range of from the lower limit of the glass transition
temperature of the principal resin constituting the vinylidene
fluoride resin to the upper limit of the melting point thereof.
More specifically, the temperature is preferably within the range
of from room temperature to 150.degree. C., more preferably from
room temperature to 120.degree. C. The voltage may be applied to
the film either while the electrodes contact both surfaces of the
film or while the electrodes are spaced apart from both surfaces
with a small gap, i.e., by a corona polarization or discharge
treatment. When the voltage is continuously applied to the film,
the corona treatment is preferred. In either method, a time of
several seconds to several tens of seconds is sufficient for the
voltage application. The above range of time is preferred because a
longer time may be applied with no trouble but will lower the
productivity.
The filter of the present invention can be obtained by gathering
the thus obtained split yarn into a form adapted for cleaning of a
fluid to be treated such as a cloth including a woven and nonwoven
cloth or a packing material inserted in a stream path.
As described hereinabove, according to the present invention, a
split yarn of a vinylidene fluoride resin which has not been
obtained heretofore is provided by treating a vinylidene fluoride
resin having an appropriate molecular weight through a combination
of specific steps. The thus obtained split yarn may have not only
superior weatherability and chemical properties such as chemical
resistance but also have a larger Young's modulus than the
polypropylene split yarn which has been substantially the sole
split yarn obtained from a synthetic resin. Furthermore, as the
vinylidene fluoride resin per se is a polar polymer, a surface
charge is extremely stably retained on the split yarn, from which
there is formed a filter excellent in effect of removing fine
charged particles from various fluids represented by air.
The present invention will be explained more specifically
hereinbelow with reference to Examples and a Comparative
Example.
EXAMPLE 1
A vinylidene fluoride homopolymer obtained by suspension
polymerization at 25.degree. C. and having an inherent viscosity of
1.1 dl/g as measured in a 0.4 g/dl solution in dimethylformamide at
30.degree. C., was melt-extruded upwardly by means of an extruder
with a circular die of 50 mm in diameter and 3.0 mm in lip
clearance at an extrusion temperature of 240.degree. C. and at an
extrusion rate of 60 g/min. The extruded tube was inflated at a
blow-up ratio of 1.5 while blowing air for cooling at a rate of
0.01 m.sup.3 /min. from an air-ring about 10 cm above the die, and
taken up at a rate of 50 m/min., namely at a draft ratio (R.sub.1)
of 670, whereby a film having a birefringence of 36.times.10.sup.-3
was obtained.
The thus inflated film was taken up about a paper tube core and
heat-treated as it was at 90.degree. C. for 2 days in a gear oven.
The film gave an average crystal width of 320 .ANG. as determined
by X-ray diffraction. Further, the film was subjected to a DSC
measurement at a temperature elevation rate of 10.degree. C./min.
in order to evaluate the crystal melting behavior, whereby two
peaks of crystal melting points were observed at 175.degree. C. and
195.degree. C., and the heat absorption area given by the melting
heat absorption peak at 195.degree. C. occupied 50% of the total
heat absorption area.
The film thus heat-treated was further stretched at 150.degree. C.
at a rate of 1.3 times. The film was further split by a
needle-planted splitter to obtain a finely divided split yarn. The
split yarn gave an average crystal width and a melting heat
absorption curve substantially the same as those obtained with
respect to the film thereof as explained above.
The split yarn was estimated to have a Young's modulus of above 500
kg/mm.sup.2 which was the value obtained with respect to the film
before splitting because it showed substantially no change in
crystal structure before and after the splitting.
EXAMPLE 2
The film after the step of stretching in Example 1 was subjected to
a corona polarization treatment by means of a corona polarization
apparatus with upper needle electrodes and lower needle electrodes
between which the film was moved at a position about 5 mm spaced
apart respectively from the upper and lower needle electrodes under
the application of a voltage of 1.5 KV.
The thus stretched and corona-treated film was split by means of a
needle-planted splitter to obtain a charged split yarn comprising
finely divided strips having an average length of 20 cm and a
section measuring 3 microns in average shorter side length and 15
microns in average longer side length.
EXAMPLE 3
The charged split yarn was formed into a nonwoven cloth of 0.05
g/cm.sup.2, which was inserted in and placed at a mid point of a
glass tube having a diameter of 30 mm and a length of 20 cm. The
glass tube was placed in front of a commercially available electric
fan so that one end of the tube was 30 cm spaced apart from the fan
and smoke of cigarette together with wind was blown into the tube.
The concentration of the smoke was observed before and after the
nonwoven cloth, whereby almost no smoke was observed in the tube
after the cloth.
COMPARATIVE EXAMPLE
The cleaning test in Example 3 was repeated by using a nonwoven
cloth filter of an isotactic polypropylene split yarn having an
average length of 20 cm and a section with an average shorter side
length of 3 microns and an average longer side length of 15
microns, whereby the concentration of the smoke was observed to be
only slightly thinned after the filter, showing apparently poorer
results, when compared with those obtained by using a split yarn of
a vinylidene fluoride resin.
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